This invention relates to a phase shifter for a broadband radio signal, such as an IF signal, and to a circuit for controlling the vector magnitude and phase reversal of such a signal.
The present invention finds particular application in a high density space diversity radio system. Such a system is used to improve reliability in microwave radio, as a solution to the problem of frequency selective fading due to multipath propagation. Reliability is improved by combining signals received by separate antennas. Circuits according to the present invention can be used to control the phase of IF signals from two different receivers with respect to each other, so that the two signals can be combined in phase or in accordance with other combining strategies.
A prior art phase shifting technique used in this type of application employs a common local oscillator for two down-converters in space diversity receivers. The local oscillator output going to the receiver which is to be phase shifted is controlled by a narrow band phase shifter. This technique uses band-sensitive and expensive radio frequency components.
Another prior method of phase shifting employs side-step conversion on an IF signal. This method is similar to the previously described one, except that it uses an up-converter and a down-converter. The local oscillator for the side-step conversion is split and, again, one side is narrow band phase shifted. This method can be used at a common IF frequency, regardless of the RF band of the receiver. Unfortunately, the method gives rise to high spurious products in the IF output and is extremely bad for high density FM receivers.